In a garbage-collected environment, sending any object a release message is hardcoded by the runtime to do nothing (very quickly). So [pool release] won’t do anything. But [pool drain] will signal the garbage collector to cleanup, and works correctly (just like release) in a non-garbage-collected environment.

Why This Still Matters on an iPhone

The iPhone doesn’t have garbage collection today. That doesn’t mean it never will. RIM and Android both support some kind of garbage collection. I’m too grizzled an Apple developer to not future proof my code, because I’ve been effected by Apple making some major runtime changes (eg. switching between PowerPC, x86, x86_64, and ARM processors). Section 3.3.1 of the iPhone SDK agreement means Apple’s runtime is the only game in town. It pays to be sure your code always plays nicely with it.

Using drain also helps your code will play nice with Mac OS X. That gives you more options to re-use and monazite it. If you decide to go the open-route, it means more people will be able to use your code.